1. Field of the Invention
The present invention generally relates to apparatus for adjusting a focal length of a lens in a video tape recorder (VTR) having a built-in camera or the like and, more particularly, to a focus adjustment apparatus that can detect a lens position with accuracy.
2. Description of the Related Art
Most of focus adjustment apparatus is small in size and generally comprises a feed screw disposed in an optical axis direction of a lens, a lens supporting member that can be moved forwardly and backwardly in accordance with the rotation of the feed screw or the like. According to the conventional focus adjustment apparatus of this type, after the lens is set to the initial position by energizing a reset switch, the feed screw is rotated by a motor to move the lens supporting member having the lens secured thereto forwardly or backwardly, thereby effecting the focus adjustment.
In this conventional focus adjustment apparatus, 30% to 50% of a load applied to the motor when the lens is being focused in the zoom lens mode is used as a pressure to enable the rack and the feed screw to be operated in a satisfactory meshed condition.
Although the above conventional focus adjustment apparatus is relatively small in size, it can endure a shock of about 100 G if the above pressure for enabling the rack to be satisfactorily meshed with the feed screw is selected to be sufficiently large. However, if the focus adjustment apparatus is increased twice in size, then the weight thereof is considerably increased, e.g., 8 times the original weight when proportioned simply. As a consequence, in proportion to the increased weight of the focus adjustment apparatus, a load applied to the motor also is increased, requiring a drive force of the motor to be increased.
If the above pressure at which the rack can be satisfactorily meshed with the feed screw upon operation is increased, then the torque of the motor that rotates the feed screw must be increased. On the other hand, if such pressure is decreased, then the rack is easily disengaged from the screw thread of the feed screw even by a very small shock. There is then the disadvantage that the rack and the feed screw is not meshed with each other (hereinafter referred to as a so-called idle running phenomenon between the rack and the feed screw).
If such idle running occurs between the rack and the feed screw, then a power switch of the focus adjustment apparatus must be turned off and then the focus adjustment apparatus must be again powered by actuating the reset switch because the focus adjustment apparatus is not provided with a position sensor that senses the lens position.